The present invention relates to a vertical-shaft furnace for the heat-treatment of metallic workpieces having a furnace housing with a heating chamber arranged therein, the heating chamber being provided with a closed wall at its upper end, while its lower end can be opened in order to enable the workpieces, which are to be treated, to be loaded into the heating chamber from below.
The term heat-treatment is used, in connection with metallic workpieces, to describe a process by which the workpieces are subjected, for improvement of their mechanical properties, to the effect of a high temperature, the temperatures employed being in the range of up to 1,000.degree. Centigrade, whereafter the workpieces are cooled down again.
If oxidation of the workpieces through the oxygen contained in the air is to be avoided, heat-treatment is performed in a protective-gas atmosphere, mostly a mixture of an inert gas such as nitrogen, and hydrogen.
The heat-treatment processes, which have become known as "tempering" and "bright annealing", include the steps of heating up the metallic workpieces and, if hardening is desired, cooling them down thereafter very rapidly in a quenching bath. The term heat-treatment further covers thermo-chemical processes where the marginal layers of the metallic materials undergo a chemical change. Such thermo-chemical treatments have been known as carburization, nitriding and nitrocarburization, respectively. The carburization process leads to the accumulation of carbon in the marginal layers of the metallic materials. The nitriding process leads to an accumulation of nitrogen in the marginal layers, and the nitro-carburization process leads to an accumulation of both, nitrogen and carbon, in the marginal layers.
In the case of vertical-shaft furnaces, the workpieces to be treated are loaded into the heating chamber in vertical direction, whereafter the heating chamber is closed gas-tight for the purpose of carrying the heat-treatment process in the heating chamber.
If very big and very heavy metallic workpieces are to be heat-treated, for example a cylindrical metallic housing of a rocket stage, having for example a height of approx. 20 m, a diameter of approx. 5 m and a weight of approx. 10 tons, there must be provided correspondingly powerful and strong means for loading such a workpiece into, and unloading it from, the heating chamber.
Another problem resides in the fact, that the lifting and lowering means employed for loading the workpiece into, and unloading it from, the vertical-shaft furnace must not remain in the heating chamber during the heat-treating process; otherwise, they would have to be heated up, too, during each heat-treating process, which would be a considerable waste of energy, and in addition they would be exposed to constant temperature changes if employed in continuous operation, which would reduce their service life. If used for thermo-chemical treatments, the means for lifting and lowering would become unserviceable after a certain number of treatments as the moving parts, in particular, would undergo heavy changes in structure due to the processing alteration of their marginal layers.
There have been known vertical-shaft furnaces which are provided with a removable lid at their upper end. When workpieces are to be loaded into the vertical-shaft furnace, the lid is opened and moved laterally clear of the opening, and the workpieces are loaded into the vertical-shaft furnace from the top, using a hoist, the furnace being in this case is closed at the bottom. Once the hoist has been removed from the vertical-shaft furnace, the lid is closed gas-tight, and the heat-treatment is carried out in the heating chamber. For removing the workpieces after completion of the heat-treatment, the lid must be opened, the hoist must be lowered into the furnace, and the workpiece, or a workpiece carrier on which the workpiece is mounted, must be picked up. If the workpieces are to be dipped into a quenching bath, the hot glowing workpieces must be lifted off the vertical-shaft oven, transported in lateral direction and then immersed into the quenching bath. This procedure is time-consuming and complicated, and in addition the workpiece gets into contact with the open air during transfer to the bath, so that oxidation processes may occur on the hot, heat-treated workpiece during such transfer, which processes may later lead to the formation of cracks and make the workpiece unserviceable. In the case of a workpiece intended to be used as a rocket stage housing, such circumstances might have fatal consequences. During the time-consuming transfer of the hot, bulky and heavy workpieces from the vertical-shaft furnace to the quenching bath, the hot workpieces may in addition start to cool partially, and this may lead to warping of the workpiece.
In the case of very big workpieces, it is an additional problem that these workpieces must be introduced into the quenching bath very rapidly in order to prevent distortions.
If, for example, a hollow-cylindrical metallic body having a height of approx. 20 m is to be introduced into a quenching bath with the cylinder axis in upright position, and if the immersion process is carried out slowly, then the portion immersed at any time in the bath will cool down rapidly already when a big portion of the body is still outside of the quenching bath and presents a considerably higher temperature. In such a case, considerable warping will be encountered.
There is further the possibility to design a heating chamber of a vertical-shaft furnace in such a way that the upper end of the heat chamber is closed by a wall, while the lower end of the heating chamber can be opened for loading the workpiece carrier with the workpieces mounted thereon into, and unloading it from, the heating chamber. The bottom of such a furnace may then be detachable or may swing to the side. If the workpieces or the workpiece carrier are to be suspended in the cavity of the heating chamber, and if the driving means are to be arranged outside the heating chamber, as mentioned before, it will be necessary to pass some mounting elements, such as rods, ropes or chains, on which the workpieces or the workpiece carrier are to be suspended, through the closed upper wall of the heating chamber. The passage must be gas-tight in order to prevent the protective gases from escaping from the heating chamber and/or oxygen contained in the air from entering the chamber, through the openings in the upper wall, during the heat-treatment process.
Another problem resides in the fact that the elements, on which the workpiece carrier is suspended and which are passed through the openings in the closed upper wall, will move past the sealing means very rapidly as the workpiece is being lowered--in the required rapid manner--from the heating chamber into a quenching bath arranged below the latter, so that the sealing means will be subjected to extremely high mechanical stresses.
A further problem is seen in the fact that the elements which are passed through the upper closed wall of the heating chamber and which carry the workpieces in the cavity of the heating chamber, are of course also heated during heating-up of the workpieces, and will then expand correspondingly. With temperature differences between room temperature and almost 1,000.degree. Centigrade, the hoists used for carrying workpieces weighing in the range of 10 tons must be provided with amply sized suspension elements for the workpieces, a fact which results in relatively big expansion values during heating-up of the workpieces.
Consequently, it is extremely difficult, and connected with many problems, to realize a permanently gas-tight connection between the opening in the upper wall of the heating chamber and the element passed therethrough, over such an important temperature range.
Special problems are encountered when open-link chains, consisting of oval links nested in each other so that they are turned alternatively by 90.degree. along the chain axis, have to be passed through the opening in the upper wall as such open-link chains do not present a uniform profile about their circumference.
It is, therefore, a first object of the invention to provide a vertical-shaft furnace which enables elements, in particular open-link chains on which workpieces are suspended, to be passed through the upper closed wall of the heating chamber in gas-tight relationship, and where this gas-tight passage will be maintained even if the elements should move in horizontal and/or vertical directions due to expansion phenomena.
The invention has further for its object to provide a vertical-shaft furnace which is easy to handle and can be operated fully automatically.
According to the invention, a vertical-shaft furnace for the heat-treatment of metallic workpieces is equipped with means for lifting a workpiece carrier from a first lower position below the lower end of the heating chamber into a second upper position inside the heating chamber, wherein the workpiece carrier is suspended on at least one flexible element which is passed through the opening in the upper wall of the heating chamber and connected to a drive outside the heating chamber, wherein the flexible element is provided with a rod-shaped section which in the second upper position of the workpiece carrier extends through the opening and is embraced in this second position by a sealing element in gas-tight relationship, wherein a wall projecting from the sealing element in downward direction surrounds the opening in the upper wall of the heating chamber, and wherein the projecting wall is immersed to float in a liquid contained in a double-walled vessel with open top which surrounds the opening in the upper wall of the heating chamber and is firmly connected to that wall at the upper end of the heating chamber.
The sealing element acts as direct gas-right seal about the circumference of the flexible element. By designing the flexible element in such a way that a rod-shaped section of the flexible element which exhibits a uniform geometry about its periphery over a certain part of its length, will occupy a position in the area of the opening when the workpiece carrier in its second upper position, it is now possible to seal the circumference of this rod-shaped area in a simple manner using conventional sealing means. Sealing is rendered particularly easy when the rod-shaped area is given a circular cross-section.
Consequently, the flexible element can be given the design of an open-link chain comprising a rod-shaped section which extends through the opening in the upper wall of the heating chamber when the workpiece carrier occupies its second, raised position. The gasket of the sealing element surrounding the rod-shaped section of the chain is then adapted to the geometry of the circumference of the rod-shaped section, to fit closely to the latter.
The floating arrangement of the wall, which projects downwardly from the sealing element, in the double-walled vessel surrounding the opening provides a gas-tight connection between the bottom face of the sealing element and the vessel surrounding the opening in the upper wall. At the same time, this arrangement makes it possible for the floating wall to move to a certain degree in horizontal direction inside the double-walled vessel. In addition, the wall is permitted to move to a certain degree in vertical direction, i.e. by exactly the length by which the wall is immersed in the liquid. This makes it possible to balance out possible variations in the position of the rod-like section of the flexible element extending through the opening, in both the horizontal and the vertical direction, by permitting the sealing element which fits tightly around the rod, or rather the wall extending downwardly therefrom, to "float" in the double-walled container in vertical and/or horizontal direction, while the gas-tight seal is permanently maintained.
The relative position between the rod-shaped section of the flexible element and the sealing element surrounding it in gas-tight relationship does not change during such movements. The projecting wall of the sealing element, which is permitted to float in the vessel, provides a gas-tight barrier between the sealing element and the outside and is capable of balancing out any positional changes between the rod-shaped sections and the opening in the wall of the heating chamber over the length by which the projecting wall of the sealing element is immersed in the liquid of the vessel, or the amount which it is permitted to move to and fro in the vessel in the horizontal direction.
Positional changes between the rod-shaped section of the flexible element and the opening in the upper wall of the heating chamber may be due, for example, to thermal expansion phenomena encountered during the heating-up phase, the expansion of the flexible element, with the workpiece suspended thereon, having a vertical and a horizontal component.
Positional changes in the horizontal position may result also when different heat-treatment processes are carried out on workpieces of different weights which may result in a greater or lesser longitudinal expansion of the flexible element, depending on the particular weight. Such longitudinal expansion may also be compensated by a vertical movement of the wall of the sealing element, which is immersed in the liquid, without any interruption of the gas-tight condition. There is then no need for compensating these positional changes by a complex control arrangement for the drive, which would require the determination of the weight of each workpiece being processed. The length of the wall in the axial direction, or the depth of immersion in the liquid, must of course be selected in a suitable way to accommodate any displacements that may be encountered in the vertical direction.
According to another embodiment of the invention, the sealing element is designed in the form of a disk provided with a central passage opening for the flexible element, and the rod-shaped section is followed, via a shoulder, by a section of a diameter larger than that of the passage opening, a gasket being arranged between the shoulder and the disk.
This feature provides the advantage that sealing between the sealing element and the rod-shaped section of the flexible element is achieved by very simply and sturdy constructional means. The flexible element is raised for this purpose until the shoulder abuts against the bottom face of the disc, thereby providing a gas-tight barrier, with the aid of the intermediate gasket. The flexible element, with the sealing element resting on its shoulder, may then be additionally raised a distance equal to the length by which the wall projecting from the bottom phase of the sealing element is immersed in the liquid. The drive of the flexible element may then be adjusted in such a way that in the unloaded condition of the flexible element, i.e. when no workpiece is suspended on it, the rod-shaped element, together with the sealing element resting on its shoulder, are raised just to a point where the wall projecting from the bottom face of the sealing element is still immersed in the liquid. Now, when a workpiece of greater or lesser weight is suspended on the flexible element, the latter is expanded by a corresponding amount (in addition to the expansion caused by the heat), the depth of the double-walled vessel and/or the length of the wall projecting from the bottom face of the sealing element being designed in such a way that the wall will continue to float in the liquid, even in the condition of maximum expansion. This guarantees at any time the gas-tight passage for the flexible element through the opening in the upper wall of the heating chamber.
According to another embodiment of the invention, the diameter of the opening in the upper closed wall of the heating chamber is such that the area of the rod-shaped section of the flexible element, which presents the largest diameter, is received in the opening at a certain lateral play.
This feature provides the advantage that even the area of greatest diameter of the flexible element will pass through the opening at a certain lateral distance from the wall surface defining the opening so that no contact will occur between the two parts during raising or lowering of the flexible element and any possible damage to the opening will be avoided. The diameter of the opening in the wall is considerably larger than that of the flexible element which extends upwardly through the passage opening in the sealing element so that, consequently, an even greater clearance remains between this portion of smaller diameter and the inner wall of the opening. It is thus possible, when lowering the workpieces from the second upper position, for example after completion of a heat-treatment process, to lower the flexible element very rapidly into the second lower position without there being any risk that the flexible element moving downward with great rapidity may damage the wall surrounding the opening. It is then also possible, for example, to pass very strong chains very rapidly through the opening in the upper wall of the heating chamber, without any risk that the wall surrounding the opening may get damaged by individual chain links getting into contact with the wall.
According to a further embodiment of the invention, the double-walled vessel surrounding the opening comprises a first inner wall encircling the opening and a second outer wall arranged at a certain distance from the said first wall, the distance between the first inner wall and the second outer wall being at least equal to the difference between the diameter of the opening in the upper wall of the heating chamber and the outer diameter of the thicker area of the rod-shaped section of the flexible element.
This feature provides the advantage that the wall projecting from the bottom face of the sealing element, viewed in the PG,13 radial direction of the opening of the wall in the heating chamber, is permitted to float in the horizontal directions by a maximum amount equal to the maximum lateral play between the flexible element and the opening.
According to another embodiment of the invention, the first inner wall is arranged immediately adjacent the edge of the circular opening in the upper wall of the heating chamber, and the wall projecting downwardly from the sealing element into the liquid of the vessel presents a cylindrical shape and occupies a central position between the first inner wall and the second outer wall when the longitudinal center axis of the flexible element extends coaxially to the longitudinal center axis of the circular opening in the upper wall.
This feature provides the advantage to render possible a horizontal floating movement of the wall in the sealing element in any radial direction.
According to a further embodiment of the invention, the vessel containing the liquid, in which the wall projecting downwardly from the sealing element is immersed in floating relationship, is connected to a recirculating device for the liquid by at least one inlet and at least one outlet, the liquid being adjusted to a predetermined temperature in the recirculating device.
This feature provides the advantage that it is now possible, for example by cooling the liquid during the heat-treatment process in the heating chamber, to maintain the structural components in the area of the passage of the flexible element through the opening in the upper wall of the heating chamber at a constant temperature. This extends the service life of the structural components, the latter being no longer exposed to the high and varying temperatures prevailing inside the heating chamber.
According to another advantageous embodiment of the invention, three flexible elements in the form of chains are provided and arranged at the corners of an imaginary equilateral triangle, relative to the cross-section of the heating chamber, the chains being connected to a single driven shaft which extends in parallel to, and at a certain distance from, one of the sides of the triangle.
This feature provides the advantage that the suspension on three chains ensures tilt-free mounting of the workpiece, relative to its horizontal position. In order to tilt a workpiece carrier, which is suspended on three chains, out of a horizontal plane, at least one of the chains would first have to be relieved from its load, a circumstance which can be excluded in view of the high weight of the workpieces. On the other hand, if the workpiece were suspended on one chain, or on two chains, the workpiece could well tilt or swing out of the horizontal plane in the area where the workpiece carrier, or the workpiece itself, is connected to the lower end of the flexible element, and in the case of a single chain there would be an additional risk of the workpiece rotating about the vertical longitudinal axis of the flexible element. Any such rotation about the vertical axis is likewise excluded when the suspension is realized by three chains arranged at the corners of an imaginary triangle. This means, however, that the orientation of the chain ends mounted at these three points of the imaginary triangle remains the same in any lifting position so that coupling of the workpiece carrier, which is suspended on three chains, to a workpiece-carrier frame or to a workpiece can be effected fully automatically, without the need to have this process supervised by an operator who will interfere manually if it should become necessary to rotate the workpiece carrier into the correct position manually. Consequently, operation of the furnace is facilitated considerably by this arrangement.
Other advantages of the invention will appear from the following description.
It is understood that the features which have been described above or which will be explained further below, may be used not only in the described combinations, but in any other combination or individually, without leaving the scope of the present invention.
The invention will now be discussed and described in more detail by way of a selected embodiment of the invention, with reference to the attached drawings.